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My teacher told that benzocyclobutadiene is both aromatic and anti-aromatic. I do not understand why. Can anyone explain?

Bicyclo[4.2.0]octa-1,3,5,7-tetraene

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Benzocyclobutadiene has an aromatic benzene ring as well as an anti-aromatic cyclobutadiene ring. This gives it the characteristics of both aromatic and anti-aromatic compounds.

Now, according to Cyclobutadiene and its related compounds, [1,p 180]:

Benzocyclobutadiene, the monobenzo derivative of cyclobutadiene, is intermediate in structure between the unstable parent compound, cyclobutadiene and it stable dibenzo analog biphenylene. Benzocyclobutadiene and its derivatives resemble cyclobutadiene rather than biphenylene in being known as only transient reaction intermediates.

Reading this, we can say that benzocyclobutadiene resembles cyclobutadiene more than biphenylene and so has a dominant anti-aromatic character. This means that like cyclobutadiene, the cyclobutadiene part of the ring undergoes dimerisation to form a more stable compound.

Note: The preview of the book referenced has an entire chapter based on just benzocyclobutadiene. If interested, you can go through it for more information

References:

  1. Cyclobutadiene and Related Compounds; M.P. Cava, M.J Mitchell; Chemistry Department, Wayne State University, Detroit 1967. ISBN 978-01-2164-450-5. [Preview]
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    $\begingroup$ Reading this you cannot say it's dominating the aromaticity. The NMR calculations referred in the book shows the alkene like character at cyclobutadiene promoting Diels Alder. You've manipulated the statements well. FYI, Cyclobutadiene can be NON aromatic as well depending on the adopted geometry. As a member of 21st century you may perform the calculations yourself at any chem-computer and find the aromaticity of phenyl ring and alkene-city of cyclobutadiene $\endgroup$
    – user96208
    Commented Aug 13, 2020 at 9:17
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    $\begingroup$ @AnindyaPrithvi If that is the case, you could submit your own answer and prove this is wrong. Unfortunately, I am currently short of such resources as you state and if a substantial proof can be given, I would be more than happy to upvote it. I stated It would be more prevalent as similar to cyclobutadiene it seems to behave more as an intermediate than be present as a compound. And so assumed it would be more antiaromatic than aromatic. There may be flaws centered around my lack of complete knowledge, so as I said above please formulate your answer. Thanks for your concern regarding my ans. $\endgroup$
    – Safdar Faisal
    Commented Aug 13, 2020 at 10:10
  • $\begingroup$ Unfortunately I do not have the time to compile resources as of now and present a full sourced answer. I only pointed out the incorrectness of your assumption using available NMR analysis, which maybe edited appropriately. As a valuable resource, you can just read about how NMR points at aromaticity and alkene nature, that will also be beneficial to point out the inaccuracy without computational/Lab analysis. $\endgroup$
    – user96208
    Commented Aug 13, 2020 at 10:29
  • $\begingroup$ pubs.acs.org/doi/10.1021/ja00168a059 $\endgroup$
    – user96208
    Commented Aug 13, 2020 at 11:04

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